The crystallographic structure of GaN grown on Si(001) substrates by metalorganic chemical vapor phase epitaxy is investigated by X-ray analysis and scanning electron microscopy. By using different buffer layer structures and growth conditions, two different crystallographic orientations could be realized. Firstly, the hexagonal GaN crystallites grow c-axis oriented on the Si(001) substrates with two rotational alignments. Secondly, the r-plane (10-12) of GaN is oriented parallel to the surface. Due to the symmetric structure of the silicon surface four rotational r-plane alignments occur. Using off-oriented Si(001) substrates one of these rotational alignments can be selected. (C) 2004 Elsevier B.V. All rights reserved.